In an increasingly competitive environment, industry is looking for ways to improve product reliability and quality, maximize efficiency to reduce costs of their manufacturing processes, and reduce product inventory. Such objectives are critically dependent upon the accurate and rapid measurement of product properties, which in many applications depends upon uniform and reproducible curing of polymers. In particular, abrasion and solvent resistance of protective coatings is diminished when a coating is incompletely cured. Similarly, performance of pressure sensitive adhesives and release coatings is particularly sensitive to incomplete and nonuniform cure.
Traditional processes of measuring extent of cure generally rely upon off-line methods, including non-destructive methods such as infrared or UV-visible absorption spectroscopy, and destructive methods such as solvent extraction, thermal analysis (glass transition temperature), and surface tack (for example ASTM-D1640-83).
A non-destructive, on-line process for monitoring degree of cure, recently disclosed in U.S. Pat. No. 4,651,011, teaches a method wherein a fluorescent material such as a dye is dissolved in a monomer, oligomer, or polymer and- can be used to monitor the degree of cure or polymerization via fluorescence anisotropy or polarization by means of an optical inspection system.
Other methods of following degree of cure by means of fluorescence spectroscopy utilize probe molecules such as those described in (a) F. W. Wang, R E. Lowry, W. H. Grant, Polymer (1984), 25, 690; (b) R. O. Loutfy in "Photophysical and Photochemical Tools in Polymer Science: Conformation, Dynamics, Morphology", NATO ASI Series, Series C, Vol. 182, M. A. Winnik, Ed., Reidel: Boston (1986) pp. 429-448; and (c) Dickinson, C. S. P. Sung, Polymer Preprints (1988), 29, 530-531; W. C. Yu, X. Y. Huang, C. S. P. Sung, Polymer Preprints (1988), 29, 532-533.
The Wang, Loutfy and U.S. Pat. No. 4,651,011 (Ors and Scarlata) methods of monitoring extent of cure require use of soluble probe molecules which are not covalently bound to the resulting polymer, providing potential environmental and measurement problems with probe "bloom".
The Wang and Loutfy methods have been shown to be useful as cure monitors only at low viscosity (less than 300 cP, reference (b) above). The Sung method requires use of special fluorescent crosslinking agents.
As to the curatives themselves, many dibenzofulvene derivatives are known in the art, as in U.S. Pat. Nos. 3,091,651 and 3,091,652, Helv. Chim. Acta (1977), 60, 1073; J. Org. Chem. (1987), 52, 688; and Ber. (1906), 39, 3062. Furthermore, it is known that certain dibenzofulvenes are either non-fluorescent or weakly fluorescent (H. Stegemeyer, Ber. Bunsenges. Phys. Chem. (1968), 72, 335-340).